- Pure ALOHA allows users to transmit whenever they have data to send.
- Senders wait to see if a collision occurred (after the whole message has send).
- If the collision occurs, each station involved waits a random amount of time then tries again.
- Systems in which multiple users share a common channel in a way that can lead to conflicts are widely known as contention systems.
Fig: In pure ALOHA, frames transmitted at completely arbitrary times
- Whenever two frames try to occupy the channel at the same time, there will a collision and both will garble.
- If the first bit of a new frame overlaps with just the last bit of a frame almost finished, both frames will totally destroy and both will have to retransmitted later.
What is the efficiency of an ALOHA channel?
- Let the ”frame time” denote the amount of time needed to transmit the standard, fixed-length frame.
- Also, assume that the infinite population of users generates new frames with mean N frames per frame time.
- If N > 1, the user community is generating frames at a higher rate than the channel can handle, and nearly every frame will suffer a collision. For reasonable throughput we would expect 0<N < 1.
- Let us further assume that the probability of k transmission attempts per frame time, old and new combined, is also Poisson, with mean G per frame time.
- Under all loads, the throughput, S, is just the offered load, G, times the probability, P0, of a transmission succeeding-that is, S = GP0, where P0 is the probability that a frame does not suffer a collision.
- The probability that k frames generated during a given frame time is given by the Poisson distribution.
- The maximum throughput occurs at G = 0.5, with S = 1/2e, which is about 0.184.